An ion detector is a type of transducer that converts ion current (ion flux, ion beam, etc.) to electrical current and thus is useful in technologies entailing the processing, transport, or manipulation of ions, such as for example mass spectrometry (MS), electronics fabrication, coating or surface treatment of articles of manufacture, etc. An ion detector is commonly employed in an MS system. Generally, an MS system converts the ionizable components of a sample material into ions and resolves (sorts, separates, or “analyzes”) the ions according to their mass-to-charge ratios, thereby producing an output of mass-discriminated ions that is transmitted to the ion detector. The information represented by the ion output received by the ion detector is thus encoded as electrical signals to enable data processing by analog and/or digital techniques. The MS system processes the resulting electrical current outputted from the ion detector as needed to produce a mass spectrum, which may entail processing/conditioning by a signal processor, storage in memory, and presentation by a readout/display means. Typically, a mass spectrum is a series of peaks indicative of the relative abundances of the detected ions as a function of mass-to-charge ratio. A trained analyst can then interpret the mass spectrum to obtain information regarding the sample material processed by the MS system.
A typical ion detector includes, as a first stage, an ion-to-electron conversion device. Ions from the mass analyzer or other type of ion source are focused toward the ion-to-electron conversion device by an appropriately applied acceleration (bias) voltage. The ion-to-electron conversion stage typically includes a surface that emits electrons in response to impingement by ions. The conversion efficiency is different for each ion mass and its energy state at the time of impact. The ion conversion stage may be followed by an electron multiplier stage. In this case, a voltage potential is impressed across the length of a containment structure of the electron multiplier. The electrical current resulting from the ion-to-electron conversion is amplified in the multiplier stage through multiplication of liberated electrons. The gain of this multiplication can be influenced by the applied voltage potential. An anode positioned at the end of the multiplier collects the multiplied flux of electrons and the resulting electrical output current is transmitted to subsequent processes. Hence, the output of an ion detector equipped with an electron multiplier is an amplified electrical current proportional to the intensity of the ion current fed to the ion detector, the ion-to-electron conversion rate, and the gain of the electron multiplier. The entrance into the electron multiplier may be biased at a fixed acceleration voltage to draw ions into the electron multiplier, as is the case of the 3×0 triple quadrapole systems available from Varian, Inc., Palo Alto, Calif. As an example, the acceleration voltage at the input of the ion detector may be ±5 kV depending on the polarity of the ions to be detected, and the gain on the signal multiplier may range up to 2 kV. This results in the output of the ion detector ranging from 3-7 kV. The output current from the ion detector can be processed as needed to yield a mass spectrum that can be displayed or printed by the readout/display means as noted above. Typically, the output current is converted to a voltage signal, digitized, and then transmitted to ground-based circuitry for further processing.
Many ion detectors are capable of detecting ions of only one polarity, that is, either positive ions or negative ions. Some ion detectors, however, have been designed to detect both positive and negative ions. Typically, the entrance into the signal multiplier is aligned on-axis with the incoming ion beam, which is disadvantageous in that neutral (uncharged) particles of no analytical value enter the ion detector and contribute to problems such as varying signal noise, reduced sensitivity, fouling, etc. Moreover, to be able to detect either positive ions or negative ions, the ion detector requires electronics that enable to polarity of the acceleration voltage to be switched. This switching requires a large voltage swing on which the gain voltage and the operating voltage of the detector's electronics ride on top. Consequently, the maximum switching speed is limited (typically 200-2000 ms) and the fast-switching circuitry required is complex and costly.
In one example of an ion detector capable of detecting either positive and negative ions, U.S. Pat. No. 4,267,448, discloses an electron multiplier inherently designed to detect positive ions. The first dynode that leads into the electron multiplier is continuously biased at −2 kV. A shutter-type acceleration electrode is positioned in front of the first dynode and can be selectively biased at either a positive or negative voltage. To detect negative ions, the acceleration electrode is biased at a positive voltage and hence operates as a conversion dynode. Negative ions impact the acceleration electrode, are converted to positive ions, and then are accelerated to the first dynode under the influence of its negative voltage bias. To detect positive ions, a high-voltage power supply connected to the acceleration electrode must be switched to a negative voltage. Another example, U.S. Pat. No. Re 33,344, similarly provides a conversion dynode in front of an electron multiplier to convert incoming negative ions to positive ions. Ion detectors such as disclosed in U.S. Pat. Nos. 4,627,448 and Re 33,344 suffer from the disadvantages noted above in that they require complex and costly switching hardware and switching between polarities causes undesirable delay. Additionally, these types of ion detectors do not adequately prevent neutral particles from entering the ion detector.
Some ion detectors have been designed to detect both positive and negative ions simultaneously. In one example, U.S. Pat. No. Re 33,344 also discloses a positively-biased conversion dynode and a negatively-biased first-stage dynode in front of a single, continuous-dynode electron multiplier. A plate is in turn positioned in front of the conversion dynode and the first-stage dynode. One aperture of the plate is aligned with the conversion dynode and another aperture of the plate is aligned with the first-stage dynode. Negative ions are attracted through the first aperture of the plate to the conversion dynode where they are converted to positive ions and subsequently flow into the electron multiplier. Positive ions are attracted through the second aperture of the plate to the first-stage dynode and subsequently flow into the remaining portion of the electron multiplier. In another example, U.S. Pat. No. 4,066,894 discloses the use of two separate ion detectors with two respective electron multipliers. The electron multipliers are arranged adjacent to each other, both in the direction of the axis of incoming ions. One ion detector is configured to detect positive ions and the other ion detector is configured to detect negative ions. Ion detectors such as disclosed in U.S. Pat. Nos. Re 33,344 and 4,066,894 also suffer from the disadvantages noted above in that they do not adequately prevent neutral particles from entering the ion detector. Moreover, they do not adequately ensure that an acceptable number of ions of a given polarity strike the corresponding first dynode and are detected.
In another example, U.S. Pat. No. 4,810,882 discloses utilizing a negatively-biased conversion electrode positioned off-axis on one side of the incoming ion flight path and a positively-biased transmission/conversion electrode positioned off-axis on the opposite side of the ion flight path. A single photomultiplier with an electron-to-photon conversion electrode is located downstream of the transmission/conversion electrode. Positive ions are deflected off-axis and strike the conversion electrode, thus releasing secondary electrons. Negative ions are deflected off-axis and strike the transmission/conversion electrode, thus releasing secondary electrons. In both cases, the secondary electrons are accelerated in the same direction through the transmission/conversion electrode toward the electron-to-photon conversion electrode of the photomultiplier. This type of ion detector is disadvantageous in that, like the other ion detectors mentioned above, the ion detector requires at least one conversion dynode. Conversion dynodes require high acceleration voltages, are prone to producing a corona discharge, and contribute to background signal noise.
Accordingly, there continues to be a need for improved ion detectors capable of detecting positive and negative ions.